This invention relates to a power transmission unit primarily intended for motor vehicle use and is particularly concerned with the control of the engagement and disengagement of friction engagement devices used in providing various drive ratios in a power transmission.
Power transmissions of the type to which this invention relates include a plurality of friction engagement devices, such as band and drum brake devices, and plate type clutches, for example. These engagement devices are normally actuated by hydraulically operated servos which, upon the supply of pressure or the relief of pressure, either cause actuation or deactuation of the respective friction engagement device for effecting speed changes in the transmission. In some cases pressure is applied to only one servo to cause actuation of the friction engagement device associated therewith and thereby effect the speed change. In other cases it is necessary to apply the hydraulic pressure to one servo and relieve the pressure in another servo to actuate one friction device and deactuate another friction device for effecting the speed change.
The timing of the supply and relief of the pressure to the servos obviously affects and controls the smoothness of the shift. If pressure is applied to two servos at the same time, one friction engagement device would cause a dragging phenomenon or fight with another friction engagement device. Automatic transmissions are initially designed with hydraulic controls that, for any given throttle opening and engine speed, are designed to cause a rotatable element, such as a drum, to increase or decrease its rate of rotation so that a graph of such rate against time follows a predetermined curve. The manufacturer or designer chooses this curve so that it represents a shift which is generally acceptable to the public, i.e., so that a shift from one speed ratio to another takes place in a predetermined period of time with as little harshness as possible.
As the various components of the friction engagement elements wear or if engine performance varies due to changes in altitude or malfunctioning of components, for example, the time in which a shift takes place, and hence the rapidity of the increase or decrease in the speed of rotating elements changes so that a graph of such rate of speed change versus time no longer follows the original predetermined curve. The time for the shift changes therefor vary over a range and may make the actual shift uncomfortable, or in an extreme condition of wear or altitude change, may be of such length that friction engagement members are subject to excessivee wear and may be excessively heated. The present invention obviates many of the objectionable features of the previously known controls for automatic transmissions.